1. Field
The disclosed embodiments relate to a method of optimizing the design of stiffened panels under stress that takes account of postbuckling on the overall scale of the fuselage.
2. Brief Description
The aspects of the disclosed embodiments include calculating the mass associated with the design criteria for panels under various loading situations. In particular, the method is intended to minimize the mass of stiffened panels taking postbuckling into account and respecting the rigidity and buckling-resistance criteria.
These disclosed embodiments come within the context of a pilot study so as to be able to evaluate, qualitatively and rapidly, new technologies and structural sensitivity studies for the purpose of shortening the development cycle for the stiffened panels that make up the fuselage of an airplane.
FIGS. 2A and 2B describe such stiffened panels. They generally consist of skins 4 reinforced in the longitudinal and orbital directions by stiffeners 3, called stringers and frames respectively. The presence of these reinforcements is intended to protect the structure from buckling phenomena and to limit crack propagation.
To obtain increasingly lightweight and strong structures, the panels resulting from optimization procedures are slender structures, and therefore liable to buckle beyond a critical load. During structural certification trials for example, generally carried out on the overall scale of the fuselage, local buckling zones are observed in the skin, forming “blisters” 6 between the stiffeners. With increasing load, these nonlinear zones may extend and cause stresses to be redistributed within the structure. For the service loads commonly encountered, these phenomena are reversible, the material remaining within the elastic region. However, they may cause stress concentrations at the bases of the stiffeners and be the cause of local disbanding, leading to general failure.
To meet the safety margins of the installation, it is therefore essential that the instability phenomena due to buckling be taken into account in the method of optimizing the design of these stiffened panels.
The problem of optimizing these large structures on the overall scale is a nonlinear problem which is complex both from its solution and from its definition, and which must take into account many criteria. Within the context of aeronautical structures for example, mass, rigidity and buckling-resistance criteria must be optimized, while still taking into account the design cost of these structures. Consequently, by dint of the large number of variables and the stresses involved, the optimization methods employed are expensive in terms of computing time.
Conventional panel design optimization methods do not generally incorporate the phenomenon of postbuckling in order to circumvent limitations in computing power and computing time.
The iterative method, in which only buckling in compression is taken into account, is also known, but this is a purely theoretical case.
Also known is the approach based on two levels—namely an overall, linear analysis level and a local, nonlinear analysis level in regions of local interest—but this two-level approach is limited owing to the fact that it makes it possible to deal only with localized nonlinearities that have no influence on the overall response.
The aspects of the disclosed embodiments provide an optimization method which is simple in its design and in its operating mode, is economical in terms of computing time and is flexible and capable of defining an optimum strategy in terms of mass in the design of a panel, whilst still taking account of the postbuckling that causes stress redistribution between frames.